Electron beam (e-beam) irradiation is widely used to crosslink a variety of different polymers for purposes of improving various properties such as hardness, tensile strength and shear strength. E-beam irradiation is also used to polymerize multifunctional oligomers and/or multifunctional monomers to make hard, scratch resistant coatings. However, due to the tendency of e-beam irradiation to effect substantial crosslinking, chain scission and grafting within treated polymers, e-beam irradiation is not generally considered an effective option for forming polymers having a high molecular weight distribution between crosslinks. Hence, other forms of irradiation, such as gamma ray and ultraviolet light, are commonly used to make polymers requiring a high molecular weight distribution between crosslinks, such as polymers having a low Tg and pressure-sensitive adhesives requiring high shear strength and high peel adhesion.
Polymers in general, and pressure-sensitive adhesives in particular, are increasingly being asked to meet demanding application requirements within ever tighter performance limitations and manufacturing specifications (e.g., polymerization with minimal use of environmentally damaging solvents).
For certain applications, such as pressure-sensitive adhesive applications, polymers need to exhibit the high tensile strength and elongation properties attainable only with high molecular weight lengths between crosslinks. Polymers having high molecular weight lengths between crosslinks also provide the necessary balancing of viscous and elastic properties required for a pressure-sensitive adhesive. In general, the adhesion, cohesion, stretchiness, and elasticity of a pressure-sensitive adhesive is dictated by the attributes of the polymer used in the composition.
Gamma irradiation is useful for sterilizing products as well as polymerizing polymers in bulk and batch processes, but is generally too slow for use in making adhesive tape in a continuous, on-web process.
Ultraviolet light is useful for polymerizing various polymers, including pressure-sensitive adhesives, without the use of a solvent. However, ultraviolet polymerization has several significant limitations. First, ultraviolet light polymerization occurs only in connection with monomeric species that absorb ultraviolet light. Since most monomers do not absorb sufficient ultraviolet light to initiate polymerization, photoinitiators are usually added to the monomers. Photoinitiators are materials that decompose into free-radicals or cations upon exposure to ultraviolet radiation. These free-radicals and/or charged molecules then initiate polymerization of the monomers. Unfortunately, photoinitiators can contribute various undesirable attributes to the resultant polymer, including yellowing of the polymer, contamination of the polymeric material with unreacted initiator and initiator fragments added to the polymer, and higher overall material costs. These contaminants can be undesirable because they can form color centers, tend to migrate to the adhesive bond surface where they inhibit performance, and are capable of initiating subsequent reactions upon exposure to ultraviolet light. The presence of such contaminants are particularly troublesome in the food and medical areas where the presence of photoinitiators is simply unacceptable due to the potential for skin irritation and toxicity.
Second, only monomeric compositions which are transparent to ultraviolet light can be effectively polymerized with ultraviolet light. Compositions that are translucent or opaque to ultraviolet light will only polymerize on the surface, since ultraviolet light cannot penetrate into the bulk of the composition. Thus, the use of many desired modifiers that absorb ultraviolet light (such as carbon black, glass beads, ultraviolet light stabilizers and colorants) and various physical structures (such as foamed adhesives and suspended particles) is limited when the monomeric composition is to be polymerized by ultraviolet light.
E-beam polymerization overcomes a number of the limitations inherent with gamma ray and ultraviolet light polymerization. However, the use of e-beam polymerization has been limited due to the inherent tendency of e-beam radiation to produce short-chain, branched, highly crosslinked polymeric structures. This phenomenon is exhibited by the tendency for e-beam polymerized pressure-sensitive adhesives to exhibit pop-off failures, frequently accompanied by low peel strength. A second limitation observed with typical e-beam polymerization techniques is a substantial concentration of residuals (e.g., unreacted monomers) remaining in the resultant polymerized product (i.e., low conversion).